I think it's perfectly all right to leave mixed comments on gift stories - the "I loved X and Y, though I'm not sure I understood the ending" kind of comments. I don't really see why everything would have to be positive - if the writer wrote the story in two days, she'll obviously know that already and can use it as an excuse. Sure, only the recipient really matters, but the comment function is there so that people can comment, and if your reaction is mostly squee with some un-squee mixed in, I think the comment should reflect that.

I think it's incredibly tacky for the recipient to point out flaws, though.